Composition for lubricating film, sliding member using the composition for lubricating film, and method of manufacturing the composition for lubricating film

ABSTRACT

A composition for a lubricating film includes molybdenum disulfide particles which are dispersed, and a binding resin, and is a composition for coating a surface of a base material made of metal with a lubricating film in which the molybdenum disulfide particles are bound together by the binding resin. The composition for a lubricating film contains 50 mass % to 70 mass % of the molybdenum disulfide particles with respect to a total amount of the molybdenum disulfide particles and the binding resin. An average particle size of the molybdenum disulfide particles is in a range of 0.1 μm to 3.0 μm.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-038150 filed on Feb. 27, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composition for a lubricating film for coating the surface of a base material with a lubricating film in which molybdenum disulfide particles are dispersed and the dispersed molybdenum disulfide particles are bound together by a binding resin, a sliding member using the same, and a method of manufacturing the same.

2. Description of Related Art

Hitherto, in order to improve sliding characteristics of a sliding member, the surface of a base material is coated with a lubricating film. Such a lubricating film contains, for example, polytetrafluoroethylene particles, graphite particles, molybdenum disulfide particles, or the like as a solid lubricant, and the particles are bound together by a binding resin (matrix resin) such as a polyamide-imide resin, a polyimide resin, or the like. As described above, by including the solid lubricant in the lubricating film, the slidabiltiy of the sliding member can be enhanced.

As a technology, for example, Japanese Patent Application Publication No. 2010-196813 (JP 2010-196813 A) suggests a sliding member in which a lubricating film formed of a resin binder and 40 mass % to 60 mass % of a solid lubricant with respect to total amount is formed on the surface of a base material. Here, the solid lubricant is formed of particles of graphite, molybdenum disulfide, or the like, and as the resin binder, a polyamide-imide resin, a polybenzimidazole resin, or a polyimide resin is used.

SUMMARY OF THE INVENTION

Here, as illustrated in FIG. 7, since a surface 91 a of a base material 91 made of metal is coated with a general lubricating film 92 such as the lubricating film disclosed in JP 2010-196813 A, the roughness of a surface 92 a of the lubricating film 92 (that is, the roughness of a surface 9 a of a sliding member 9) is likely to be dependent on the roughness of the surface 91 a of the base material 91.

However, in a case where the average particle size of particles 93 that act as the solid lubricant contained in the lubricating film 92 (the average particle size of the particles contained in the composition for a lubricating film) is large, the surface roughness of the lubricating film 92 may become coarser than the surface roughness of the base material 91. In addition, even in a case where the amount of the particles 93 (the amount of particles contained in the composition for a lubricating film) is increased in order to enhance seizure resistance, the same phenomenon occurs.

Accordingly, even when the surface 91 a of the base material 91 of the sliding member is further smoothened, the sliding surface of the sliding member is less likely to become smooth. As a result, there may be cases where it is difficult to obtain low friction sliding characteristics.

The present invention provides a composition for a lubricating film which reduces the surface roughness of a lubricating film formed on the surface of a base material of a sliding member and allows the surface of the lubricating film to be easily smoothened during sliding, a sliding member using the same, and a method of manufacturing the same.

According to an aspect of the present invention, a composition for a lubricating film includes molybdenum disulfide particles which are dispersed, and a binding resin. The composition for a lubricating film is a composition for coating a surface of a base material made of metal with a lubricating film in which the molybdenum disulfide particles are bound together by the binding resin. The composition for a lubricating film contains 50 mass % to 70 mass % of the molybdenum disulfide particles with respect to a total amount of the molybdenum disulfide particles and the binding resin. An average particle size of the molybdenum disulfide particles is in a range of 0.1 μm to 3.0 μm.

According to the aspect, since 50 mass % to 70 mass % of the molybdenum disulfide particles are contained, initial fitting characteristics are enhanced, and thus, the surface of the lubricating film is easily smoothened during sliding. Furthermore, since the average particle size of the molybdenum disulfide particles is 0.1 μm to 3.0 μm, the particles size is reduced, and thus, the surface of the lubricating film is smoothened.

Accordingly, when the sliding member is used, the surface of the lubricating film is easily worn, and thus, the initial fitting characteristics of the lubricating film (sliding member) can be enhanced.

In the aspect, the binding resin may be a polyamide-imide resin.

In the aspect, particle binding properties, adhesion to the surface of the base material, heat-resistance of the lubricating film, and the like are considered.

In the aspect, there is provided a sliding member, in which the surface of the base material is coated with the lubricating film formed of the composition for a lubricating film.

In the aspect, a plurality of dimples may be formed on the surface. A projecting valley depth of the surface may be in a range of 0.4 μm to 1.0 μm, and a center-line average roughness of the surface may be in a range of 0.4 μm to 1.0 μm.

As is apparent from an experiment by the inventors, which will be described later, by establishing the relationship between the projecting valley depth Rvk and the center-line average roughness Ra of the surface of the lubricating film, that is, the sliding surface of the sliding member, low friction characteristics can be realized while enhancing the initial fitting characteristics of the sliding surface of the sliding member.

In the aspect, the sliding member may be a piston of an internal-combustion engine. In addition, the lubricating film may be formed on a surface of a skirt section of the piston.

In the aspect, there is provided a method of manufacturing a sliding member, in which a surface of a base material is coated with a lubricating film formed of the composition for a lubricating film.

In the aspect, the method may include forming a plurality of dimples, and forming the lubricating film on the surface on which the dimples are formed, with the composition for a lubricating film. The dimples are formed by propelling hard particles against the surface of the base material through shot peening to allow the surface to have a projecting valley depth in a range of 1.5 μm to 2.0 μm and a center-line average roughness in a range of 1.5 μm to 2.0 μm. The hard particles are harder than the base material.

Accordingly, the sliding member having the above-described characteristics can be easily manufactured.

In the aspect, there is provided a method of manufacturing the sliding member as a piston, according to the method of manufacturing a sliding member. The method may include, after forming a plurality of dimples on a surface of a skirt section of the piston, coating the surface of the skirt section, on which the dimples are formed, with the lubricating film.

According to the aspects of the present invention, the surface roughness of the lubricating film formed on the surface of the base material of the sliding member is reduced, and the surface of the lubricating film is easily smoothened during sliding.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1A is a schematic view illustrating a method of manufacturing a sliding member according to an embodiment, and is a view illustrating a surface treatment of a base material;

FIG. 1B is a schematic view illustrating the method of manufacturing a sliding member according to the embodiment, and is a view illustrating the surface-treated base material;

FIG. 1C is a schematic view illustrating the method of manufacturing a sliding member according to the embodiment, and is a view illustrating the sliding member in which the surface of the base material is coated with a lubricating film;

FIG. 2 is a view illustrating the surface roughnesses of sliding members according to Examples 1 to 3 and Comparative Examples 1 to 5;

FIG. 3 is a view illustrating a wear test of the sliding members according to Examples 1 to 3 and Comparative Examples 1 to 5;

FIG. 4 is a view illustrating the wear amounts of the lubricating films of the sliding members according to Examples 1 to 3 and Comparative Examples 1 to 5;

FIG. 5 is a view illustrating the relationship between rotation frequency-BMEP and FMEP when pistons according to Examples 4 and 5 and Comparative Example 6 are used;

FIG. 6 is a view illustrating the relationship between rotation frequency-BMEP and FMEP when the pistons according to Example 5 and Comparative Example 7 are used; and

FIG. 7 is a schematic view illustrating a sliding member according to the related art.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described using FIGS. 1A, 1B, and 1C. FIGS. 1A, 1B, and 1C are schematic views illustrating a method of manufacturing a sliding member according to the embodiment, and FIG. 1A is a view illustrating a surface treatment of a base material. FIG. 1B is a view illustrating the surface-treated base material. FIG. 1C is a view illustrating the sliding member in which the surface of the base material is coated with a lubricating film.

Regarding a composition for a lubricating film, a composition for a lubricating film according to this embodiment is composition for coating a surface 10 b of a base material 10 made of metal, with a lubricating film 20, as illustrated in FIG. 1C. The composition for a lubricating film contains molybdenum disulfide particles and a binding resin (resin binder) for binding the particles.

The composition for a lubricating film contains 50 mass % to 70 mass % of the molybdenum disulfide particles with respect to the total amount of the molybdenum disulfide particles and the binding resin. In addition, the average particle size of the molybdenum disulfide particles is in a range of 0.1 μm to 3.0 μm.

Here, “particle size” mentioned in the specification represents a volume cumulative average particle size (D₅₀) measured by a laser diffraction/scattering type particle size distribution measurement method, “center-line average roughness Ra” represents a value measured according to JIS B 0601-1994, and “projecting valley depth (oil accumulation depth) Rvk” represents a value measured according to JIS B 0601-2001.

In this embodiment, by allowing 50 mass % to 70 mass % of the molybdenum disulfide particles to be contained and allowing the average particle size of the molybdenum disulfide particles to be 0.1 μm to 3.0 μm, the surface roughness of the lubricating film 20 formed on the surface 10 b of the base material 10 of a sliding member 1, which will be described later, can be the same degree of surface roughness as the roughness of the surface 10 b of the base material 10. Accordingly, the surface of the lubricating film 20 can be smoothened. Furthermore, when the sliding member 1 is used, the surface of the lubricating film 20 is easily worn. Therefore, the initial fitting characteristics of the lubricating film 20 (the sliding member 1) can be enhanced.

Here, in a case where the amount of the molybdenum disulfide particles is more than 70 mass % or in a case where the average particle size of the molybdenum disulfide particles is greater than 3.0 μm, the surface roughness of the formed lubricating film 20 is increased, and the smoothening of a sliding surface 1 a of the sliding member 1 may be impeded.

On the other hand, in a case where the amount of the molybdenum disulfide particles is less than 50 mass %, in an initial fitting stage, the lubricating film 20 is less likely to be worn, and the initial fitting characteristics of the sliding member 1 are degraded. It is difficult to manufacture molybdenum disulfide particles having an average particle size of smaller than 0.1 μm.

Here, molybdenum disulfide particles having an average particle size in a range of 0.1 μm to 3.0 μm can be easily obtained by crushing and classifying commercially available molybdenum disulfide particles or the like. The average particle size of molybdenum disulfide particles by which the above-described effects can be further expected is 0.5 μm to 1.5 μm.

The binding resin is preferably a heat-resistant resin capable of binding molybdenum disulfide particles. Examples thereof include a resin selected from the group consisting of an epoxy resin, a phenol resin, a polyamide resin, and a polyamide-imide resin, or a polymer alloy containing one type or two or more types thereof. Among these, the polyamide-imide resin has excellent heat resistance, is less likely to deteriorate even when used for various types of lubricating oil, and has good friction and wear characteristics.

The composition for a lubricating film used for forming the lubricating film can be prepared by preparing the molybdenum disulfide particles and the binding resin at the above-described mixing ratio, and stirring and mixing the resultant with an appropriate organic solvent as the solvent using a kneader or the like. During the manufacturing of the composition for a lubricating film, it is preferable to mix the molybdenum disulfide particles in a state in which the binding resin is preliminarily dissolved in the organic solvent.

As the organic solvent, a volatile polar aprotic solvent or the like is appropriately used. Specific examples include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone, esters such as methyl acetate and ethyl acetate, aromatic hydrocarbons such as xylene and toluene, organic halogen compounds such as methyl chloroform, trichloroethylene, and trichlorotrifluoroethane, N-methyl-2-pyrrolidone (NMP), methyl iso-pyrrolidone (MIP), dimethylformamide (DMF), and dimethylacetamide (DMAC). N-methyl-2-pyrrolidone (NMP) may be particularly preferably used. A non-polar solvent may also be used as the organic solvent.

Furthermore, the composition for a lubricating film may also contain a dispersant which promotes the dispersion of a solid lubricant, a silane coupling agent which enhances the adhesion of a resin to the solid lubricant or the adhesion of a resin to the base material, and a surfactant which controls the surface tension. Moreover, an appropriate amount of additives (a suspending agent, a thickener, an anti-foaming agent, a leveling agent, and the like) for improving storage stability, film fitting properties, and the like may be added without departing from the object of the present invention.

For the sliding member and the manufacturing method, first, a base material made of metal is prepared. The base material is preferably a base material made of type or more types of metals selected from aluminum, iron, copper, nickel, chromium, and titanium, and may also be a base material made of an alloy containing the metals. As desired, a metal base material subjected to a surface treatment such as alkali degreasing may also be used.

Here, the surface roughness of the surface of the base material made of metal may be appropriately set depending on the use environment. In this embodiment, when the lubricating film is formed of the composition for a lubricating film, which will be described later, the surface roughness of the lubricating film (the surface roughness of the sliding member) becomes a value close to the surface roughness of the base material.

For example, the lubricating film may be formed by applying the composition for a lubricating film to the surface (cut or ground surface) of the base material made of metal illustrated in FIG. 1A. However, in this embodiment, a base surface treatment is performed to smoothen the surface of the base material 10. Specifically, as illustrated in FIG. 1A, hard particles 40, which are harder than the base material 10, are propelled against the surface 10 a of the base material 10 through shot peening. By the shot peening, the surface 10 b in which dimples are formed to allow the base material 10 to have a projecting valley depth Rvk in a range of 1.5 μm to 2.0 μm and a center-line average roughness Ra in a range of 1.5 μm to 2.0 μm is formed (see FIG. 1B). Accordingly, the surface 10 b of the base material 10, which is optimal for exhibiting the effects of the composition for a lubricating film according to this embodiment, can be obtained.

For example, in a case where the base material is made of an aluminum alloy (Al—Si-based alloy), the hard particles 40 of an iron-based alloy having an average particle size of 20 μm to 300 μm are used during the shot peening. First, the above-mentioned hard particles 40 are propelled against the surface 10 a of the base material 10 under the condition of a predetermined arc height value in an atmosphere in which oxygen is present (for example, in the air).

As described above, the surface 10 b of the base material 10 in which a plurality of dimples having the above-described surface properties are formed can be obtained. In addition, the projecting valley depth Rvk and the center-line average roughness Ra can be adjusted by changing the shot diameter, shot propelling conditions during the propelling process, and the like.

Next, the surface 10 b of the base material 10 in which the dimples are formed is coated with the composition for a lubricating film to form the lubricating film 20. Coating of the surface 10 b of the base material 10 may be performed by well-known methods such as brush coating, spray coating, roll coating, coating using screen printing, knife coating, coating according to a padding method, and immersion coating. Industrially, coating may also be performed by air spraying.

After the composition for a lubricating film is applied to the surface 10 b of the base material 10, a heating treatment is performed to remove the organic solvent from the composition, such that the surface of the base material 10 can be coated with the lubricating film 20 as illustrated in FIG. 1C.

The projecting valley depth Rvk of the surface of the obtained lubricating film 20 is in a range of 0.4 μm to 1.0 μm, and the center-line average roughness Ra of the surface of the lubricating film 20 is in a range of 0.4 μm to 1.0 μm. As a result, by allowing the relationship between the projecting valley depth Rvk and the center-line average roughness Ra of the sliding surface 1 a of the sliding member 1 to be in the above-described range, the sliding surface 1 a can be enhanced in the property of holding the lubricating oil in a state in which the surface roughness of the sliding surface 1 a is lower than that in the related art. Accordingly, low friction characteristics can be realized while enhancing the initial fitting characteristics of the sliding surface 1 a of the sliding member 1.

Here, the thickness of the lubricating film is arbitrary, but is preferably 6 μm to 16 μm. By allowing the thickness to be in the above-described range, in a case where the molybdenum disulfide particles having the above-mentioned average particle size are contained in the above-described range, the lubricating film can be smoothened, so that the sliding surface is allowed to be dependent on the surface of the base material and the sliding surface is allowed to be in the above-described range. When the composition for a lubricating film is applied to the sliding surface of the skirt section of a piston and the lubricating film is formed, a reduction in the thermal conductivity of the piston can be suppressed.

In the related art, when the surface of the base material is polished by a grindstone or the like, the surface has striation marks and thus the actual surface pressure is increased. Accordingly, the initial fitting characteristics cannot be enhanced by allowing a large amount of the molybdenum disulfide particles to be mixed. However, in this embodiment, the surface 10 b of the base material 10 smoothened by the base surface treatment described above is coated with the lubricating film 20, and the actual surface pressure of the sliding surface la is reduced by reducing the average particle size of the molybdenum disulfide particles contained in the lubricating film 20, thereby suppressing wear. Accordingly, even when a larger amount of the molybdenum disulfide particles is mixed, the wear resistance is ensured without the separation or the like of the particles. Therefore, the initial fitting characteristics of the lubricating film 20 are enhanced and a smooth sliding surface can be formed at an early stage.

Hereinafter, Examples of the present invention will be described.

EXAMPLE 1

As the base material of the sliding member, an aluminum alloy casting (AC8B in JIS standards) having a size of 15.7 mm×6.5 mm×10.1 mm and a surface roughness Rz of 0.8 μm (corresponding to an Ra of 0.2 μm) was prepared, and the surface of the base material was subjected to alkali degreasing.

Subsequently, molybdenum disulfide particles having an average particle size of 2 μm were prepared as the solid lubricant, and a polyamide-imide resin (PAI) dissolved in NMP as the organic solvent was prepared as a resin for binding the particles. 50 mass % of the molybdenum disulfide particles and 50 mass % (mass % of the resin excluding the organic solvent) of the polyamide-imide resin were blended as shown in Table 1, and were mixed, thereby producing a composition for a lubricating film. Subsequently, the base material was preliminarily heated under the condition of 80° C.×30 minutes, the composition for a lubricating film was applied to the surface of the base material, the resultant was heated under the condition of 180° C.×90 minutes, thereby forming a lubricating film having a thickness of 13 μm on the base material.

Here, the average particle size of the molybdenum disulfide particles was measured by a measurement method according to PIDS (polarization intensity differential scattering) using a laser diffraction/scattering type particle size distribution measurement apparatus (Microtrac MT300 made by Nikkiso Co., Ltd.). The average particle size is a volume cumulative average particle size D₅₀ of the molybdenum disulfide particles measured by NMP. In addition, the ratios (mass %) of the molybdenum disulfide particles and the polyamide-imide resin were measured through weighing.

EXAMPLE 2

A sliding member was produced in the same manner as in Example 1. The difference from Example 1 is that as shown in Table 1, 60 mass % of the molybdenum disulfide particles and 40 mass % of the polyamide-imide resin were blended to produce a composition for a lubricating film.

EXAMPLE 3

A sliding member was produced in the same manner as in Example 1. The difference from Example 1 is that as shown in Table 1, 70 mass % of the molybdenum disulfide particles and 30 mass % of the polyamide-imide resin were blended to produce a composition for a lubricating film.

COMPARATIVE EXAMPLE 1

A sliding member was produced in the same manner as in Example 1. The difference from Example 1 is that as shown in Table 1, with respect to 70 mass % of the polyamide-imide resin, as the solid lubricant, 20 mass % of molybdenum disulfide particles having an average particle size of 7 μm, 7 mass % of graphite particles having an average particle size of 2.5 μm, and 3 mass % of polytetrafluoroethylene (PTFE) particles having an average particle size of 4 μm were blended to produce a composition for a lubricating film.

COMPARATIVE EXAMPLE 2

A sliding member was produced in the same manner as in Example 1. The difference from Example 1 is that as shown in Table 1, 40 mass % of the molybdenum disulfide particles and 60 mass % of the polyamide-imide resin were blended to produce a composition for a lubricating film.

COMPARATIVE EXAMPLE 3

A sliding member was produced in the same manner as in Example 1. The difference from Example 1 is that as shown in Table 1, 80 mass % of the molybdenum disulfide particles and 20 mass % of the polyamide-imide resin were blended to produce a composition for a lubricating film.

COMPARATIVE EXAMPLE 4

A sliding member was produced in the same manner as in Example 1. The difference from Example 1 is that as shown in Table 1, 50 mass % of the graphite particles used in Comparative Example 1 and 50 mass % of the polyamide-imide resin were blended to produce a composition for a lubricating film.

COMPARATIVE EXAMPLE 5

A sliding member was produced in the same manner as in Example 1. The difference from Example 1 is that as shown in Table 1, 50 mass % of the PTFE particles used in Comparative Example 1 and 50 mass % of the polyamide-imide resin were blended to produce a composition for a lubricating film.

<Measurement of Surface Roughness of Lubricating Film>

The surface roughnesses (center-line average roughnesses Ra) of the lubricating films of the sliding members according to Examples 1 to 3 and Comparative Examples 1 to 5 were measured. The center-line average roughness Ra is a value measured according to JIS B 0601-1994. The results are shown in Table 1 and FIG. 2. FIG. 2 is a view illustrating the surface roughnesses of the sliding members according to Examples 1 to 3 and Comparative Examples 1 to 5. Broken line shown in FIG. 2 is the surface roughness of the base material.

<Initial Fitting Test>

As shown in FIG. 3, a block test piece 51 corresponding to the sliding member of Examples 1 to 3 an Comparative Examples 1 to 5 described above, a ring test piece 52, and a lubricating oil 53 were combined, and a friction and wear test (block-on-ring test (LFW-1 test), made by Falex Corporation) was conducted.

Specifically, as the ring test piece 52, a test piece which is made of gray cast iron (FC250 in JIS standards) and has an outer diameter of 35 mm, a width of 8.8 mm, and a surface roughness Rz of 0.8 μm was prepared. The lubricating oil 53 was poured into an oil bath 54 to allow a portion of the ring test piece to be dipped into the lubricating oil 53, and the ring test piece 52 was rotated at a peripheral speed of 0.11 m/s while the oil temperature was held at 80° C. to form an oil film on the surface of the ring test piece 52. A continuous test was conducted for 5 minutes by allowing the block test piece 51 to come into contact with the outer peripheral surface of the ring test piece 52 under a load of 22.5 N.

The wear depth of the lubricating film of the sliding member, which was the block test piece 51, was measured after the end of the test, and was represented as a wear amount. As the lubricating oil, base oil (commercially available engine oil in the SAE viscosity grade of 0W-20) was used. The results are shown in Table 1 and FIG. 4. FIG. 4 is a view illustrating the wear amounts of the lubricating films of the sliding members according to Examples 1 to 3 and Comparative Examples 1 to 5. Broken line shown in FIG. 4 is the lower limit of the wear amount determined from the experience of the inventors or the like to provide good initial fitting characteristics.

TABLE 1 Solid lubricant Surface roughness Wear Type of PAI Ra depth particles mass % mass % μm μm Example 1 MoS₂ 50 50 0.13 8.1 Example 2 MoS₂ 60 40 0.18 8.2 Example 3 MoS₂ 70 30 0.20 9.5 Comparative MoS₂ 20 70 0.45 6.6 Example 1 Graphite 7 PTFE 3 Comparative MoS₂ 40 60 0.09 4.8 Example 2 Comparative MoS₂ 80 20 0.31 11.5 Example 3 Comparative Graphite 50 50 0.47 9.7 Example 4 Comparative PTFE 50 50 0.56 10.1 Example 5

(Result 1)

As shown in FIG. 2 and Table 1, the surface roughnesses of the lubricating films formed of the compositions for a lubricating film according to Examples 1 to 3 were lower than those of Comparative Examples 1, 3, 4, and 5.

That is, in a case where the lubricating films were formed of the compositions for a lubricating film according to Comparative Examples 1, 3, 4, and 5, the surface roughness of the lubricating film is coarsened. It is thought that this is because in the case of Comparative Examples 1, 4, and 5, the average particle sizes of the solid lubricants contained in the compositions for a lubricating film are greater than those of Examples 1 to 3. On the other hand, the case of Comparative Example 3 is caused by that the amount of the solid lubricant (molybdenum disulfide particles) contained in the composition for a lubricating film is more than those of Examples 1 to 3.

On the other hand, as shown in FIG. 4 and Table 1, the wear amounts of the lubricating films of the sliding members according to Examples 1 to 3 and Comparative Examples 3 to 5 are more than those of Comparative Examples 1 and 2. It is thought that this is because the amounts of the molybdenum disulfide particles of the lubricating films of the sliding members according to Examples 1 to 3 and Comparative Examples 3 to 5 are more than those of Comparative Examples 1 and 2. However, since the surface roughnesses of the lubricating films of Comparative Examples 3 to 5 are higher than the surface roughnesses of those of Examples 1 to 3, the ability to form an oil film is low. Therefore, it can be said that the lubricating films of the sliding members according to Examples 1 to 3 have a higher ability to form an oil film than those of Comparative Examples 1 to 5 and have good initial fitting characteristics.

From the above description, it is thought that when 50 to 70 mass % of the molybdenum disulfide particles are contained in the composition for a lubricating film and the average particle size thereof is in a range of 0.1 μm to 3.0 μm, the lubricating film has a surface roughness corresponding to the surface roughness of the base material (smoothness is provided) and thus good initial fitting characteristics are provided.

EXAMPLE 4

As the sliding member, a piston (base material) of an internal-combustion engine formed of an AC8-based aluminum alloy casting was prepared, and cutting work was performed on the surface of the skirt section of the piston. Accordingly, a plurality of striation marks were formed on the surface of the skirt section. Next, the projecting valley depth (oil accumulation depths) Rvk at a plurality of points on the surface of the skirt section was measured according to JIS B 0601-2001, and the center-line average roughness Ra thereof was measured according to the above-described method. As a result, the maximum value of the projecting valley depth Rvk was 1.0 μm and the minimum value thereof was 0.2 μm (the projecting valley depth Rvk was 0.2 μm to 1.0 μm), and the maximum value of the center-line average roughness Ra was 4.8 μm and the minimum value thereof was 2.5 μm (the center-line average roughness Ra was 2.5 μm to 4.8 μm).

A lubricating film was formed on the surface using the same composition for a lubricating film as that in Example 1 under the same conditions as those of Example 1. The projecting valley depth Rvk and the center-line average roughness Ra at the plurality of points on the surface (sliding surface) of the lubricating film after being formed were measured by the same method as the above-described method. As a result, the projecting valley depth Rvk was in a range of 0.2 μm to 1.0 μm, and the center-line average roughness Ra was in a range of 2.5 μm to 4.8 μm.

EXAMPLE 5

In the same manner as in Example 4, a piston (base material) of an internal-combustion engine formed of an AC8-based aluminum alloy casting was prepared, and a surface treatment was performed on the surface of the skirt section of the piston. Specifically, using hard particles which are harder than those of the base material (specifically, shot formed of an iron-based material having an average particle size of 50 μm), a plurality of dimples were formed on the surface of the skirt section through shot peening.

Subsequently, the projecting valley depth (oil accumulation depth) Rvk and the center-line average roughness Ra at the plurality of points on the surface of the skirt section were measured. As a result, the maximum value of the projecting valley depth Rvk was 2.0 μm and the minimum value thereof was 1.5 μm (the projecting valley depth Rvk was 1.5 μm to 2.0 μm), and the maximum value of the center-line average roughness Ra was 2.0 μm and the minimum value thereof was 1.5 μm (the center-line average roughness Ra was 1.5 μm to 2.0 μm).

A lubricating film was formed on the surface using the same composition for a lubricating film as that in Example 1 under the same conditions as those of Example 1. The projecting valley depth Rvk and the center-line average roughness Ra at the plurality of points on the surface (sliding surface) of the lubricating film after being formed were measured by the same method as the above-described method. As a result, the projecting valley depth Rvk was in a range of 0.4 μm to 1.0 μm, and the center-line average roughness Ra was in a range of 0.4 μm to 1.0 μm.

COMPARATIVE EXAMPLE 6

In the same manner as in Example 4, a piston (base material) of an internal-combustion engine formed of an AC8-based aluminum alloy casting was prepared, and cutting work was performed on the surface of the skirt section of the piston as in Example 4. Accordingly, a plurality of striation marks were formed on the surface of the skirt section. The projecting valley depth Rvk and the center-line average roughness Ra of the surface of the skirt section were in the same ranges as those described in Example 4.

A lubricating film was formed on the surface using the same composition for a lubricating film as that in Comparative Example 1 under the same conditions as those of Comparative Example 1. The projecting valley depth Rvk and the center-line average roughness Ra at the plurality of points on the surface (sliding surface) of the lubricating film after being formed were measured. The projecting valley depth Rvk and the center-line average roughness Ra of Comparative Example 6 were in the same ranges as those described in Example 4.

COMPARATIVE EXAMPLE 7

In the same manner as in Example 5, a piston (base material) of an internal-combustion engine formed of an AC8-based aluminum alloy casting was prepared, and for the surface of the skirt section of the piston, a plurality of dimples were formed on the surface of the skirt section through shot peening. The projecting valley depth Rvk and the center-line average roughness Ra of the surface of the skirt section were in the same ranges as those described in Example 5.

A lubricating film was formed on the surface using the same composition for a lubricating film as that in Comparative Example 1 under the same conditions as those of Comparative Example 1. The projecting valley depth Rvk and the center-line average roughness Ra at the plurality of points on the surface (sliding surface) of the lubricating film after being formed were measured. The projecting valley depth Rvk and the center-line average roughness Ra of Comparative Example 7 were in the same ranges as those described in Example 5.

<Actual Machine Test>

Using the pistons according to Examples 4 and 5 and Comparative Examples 6 and 7, an actual machine test was conducted. Specifically, the piston had an emission amount of about 660 cc, a cylinder bore diameter of 94 mm, and a stroke of 95 mm, 0W-20 was used as the engine oil, and an oil/water temperature was set to 80° C.±1° C. Subsequently, while increasing a rotation frequency in stages to 2000 rpm, and changing a brake mean effective pressure (BMEP), a friction mean effective pressure (FMEP) was measured. The results are shown in FIGS. 5 and 6. FIG. 5 is a view illustrating the relationship between rotation frequency-BMEP and FMEP when the pistons according to Examples 4 and 5 and Comparative Example 6 are used. FIG. 6 is a view illustrating the relationship between rotation frequency-BMEP and FMEP when the pistons according to Example 5 and Comparative Example 7 are used.

(Result 2)

As shown in FIG. 5, the friction mean effective pressures (FMEP) of Examples 4 and 5 are lower than that of Comparative Example 6, and the friction mean effective pressure (FMEP) of Example 5 was lower than that of Example 4. In addition, as shown in FIG. 6, the friction mean effective pressure (FMEP) of Example 5 was lower than that of Comparative Example 7.

From this, it is thought that since the composition for a lubricating film according to Example 1 was used in Examples 4 and 5, even when the molybdenum disulfide particles were contained therein, the lubricating film could be formed without allowing a surface roughness to be greater than the surface roughness of the base material. Accordingly, it is thought that since the lubricating films of Examples 4 and 5 easily form oil films, the FMEPs of Examples 4 and 5 became lower than that of Comparative Example 6.

Particularly, in the case of the lubricating film of Example 5, the projecting valley depth (oil accumulation depth) Rvk thereof is greater than that in Example 4 regardless of that the center-line average roughness Ra thereof is small. Accordingly, it is thought that in the case of Example 5, a thicker oil film is more easily formed than in Example 4, and the FMEP in Example 5 became lower than that in Example 4. However, it is thought that in the case of Comparative Example 7, since the composition for a lubricating film according to Comparative Example 1 was used, it is difficult to be dependent on the surface properties of the base material, and the FMEP in Comparative Example 7 became greater than that in Example 5.

Furthermore, when a plurality of the surface roughnesses of the lubricating film of the skirt section after the actual machine test were measured, the surface roughness of the lubricating film according to Example 4 (for example, an Ra of 2.1 μm) was smaller than that of Comparative Example 6 (for example, an Ra of 2.3 μm). Similarly, the surface roughness of the lubricating film according to Example 5 (for example, an Ra of 0.72 μm) was smaller than that of Comparative Example 7 (for example, an Ra of 1.01 μm). The relationship between the surface roughnesses was the same as the relationship therebetween before the actual machine test. From this result and the result of the FMEP described above, it is thought that the sliding members according to Examples 4 and 5 have higher initial fitting characteristics than those of Comparative Examples 6 and 7.

While the embodiments of the present invention have been described in detail, the present invention is not limited to the above-described embodiments, and various changes in design can be made without departing from the spirit of the present invention described in the appended claims and the specification. 

What is claimed is:
 1. A composition for a lubricating film, comprising: molybdenum disulfide particles which are dispersed; and a binding resin, wherein the composition for the lubricating film is a composition for coating a surface of a base material made of metal with a lubricating film in which the molybdenum disulfide particles are bound together by the binding resin, the composition for the lubricating film contains 50 mass % to 70 mass % of the molybdenum disulfide particles with respect to a total amount of the molybdenum disulfide particles and the binding resin, and an average particle size of the molybdenum disulfide particles is in a range of 0.1 μm to 3.0 μm.
 2. The composition for the lubricating film according to claim 1, wherein the binding resin is a polyamide-imide resin.
 3. A sliding member, in which the surface of the base material is coated with the lubricating film formed of the composition for a lubricating film according to claim
 1. 4. The sliding member according to claim 3, wherein a plurality of dimples are formed on the surface, a projecting valley depth of the surface is in a range of 0.4 μm to 1.0 μm, and a center-line average roughness of the surface is in a range of 0.4 μm to 1.0 μm.
 5. The sliding member according to claim 3, wherein the sliding member is a piston of an internal-combustion engine, and the lubricating film is formed on a surface of a skirt section of the piston.
 6. A method of manufacturing a sliding member, in which a surface of the base material is coated with a lubricating film formed of the composition for a lubricating film according to claim
 1. 7. The method according to claim 6, comprising: forming a plurality of dimples, the dimples being formed by propelling hard particles against the surface of the base material through shot peening to allow the surface to have a projecting valley depth in a range of 1.5 μm to 2.0 μm and a center-line average roughness in a range of 1.5 μm to 2.0 μm, the hard particles being harder than the base material; and forming the lubricating film on the surface on which the dimples are formed, with the composition for a lubricating film.
 8. A method of manufacturing a piston using the sliding member as the piston, according to the method of manufacturing a sliding member according to claim 7, the method further comprising: after forming the plurality of dimples on the surface of a skirt section of the piston, coating the surface of the skirt section, on which the dimples are formed, with the lubricating film. 